Bradley Smoker PLC-5 user manual Priority Scheduling for Interrupts and MCPs

Programming Considerations 13-11

Deciding When to Use Special Routines

If a Portion of Logic
Should Execute	Example	Use	By Doing the Following:
Immediately on	Restart the system after	Power-Up Routine	Create a separate file for a controlled start-up procedure for the first time
detecting conditions	the system has been shut		you start a program or when you start a program after system down time.
that require a startup	down		The controller executes the power-up routine to completion.
Immediately on detecting	Shut down plant floor	Fault Routine	Create a separate file for a controlled response to a major fault. The first
a major fault	devices safely upon		fault detected determines which fault routine is executed. The controller
	detecting a major fault		executes the fault routine to completion. If the routine clears the fault, the
			controller resumes the main logic program where it was interrupted. If
	or		not, the controller faults and switches to program mode.
	Send critical status to a
	supervisory controller
	via DH+ after detecting a
	major fault
At a specified time	Monitor machine position	Selectable Timed	Create a separate program file and specify the interrupt time interval. The
interval	every 250ms and	Interrupt (STI)	controller interrupts the main logic program at the specified interval, runs
	calculate the average		the STI to completion, then resumes the main logic program where it left
	rate-of-change		off.
	or		The controller interrupts the main logic program at the specified interval
			and runs the STIs. When a block-transfer instruction to remote I/O is
	Take a measurement and		encountered in an STI, the controller resumes execution of lower priority
			programs (main logic program) until the block-transfer is completed.
	compare it with a		When this occurs and you want your STI to run to completion before
	standard every 1.0		returning to the main logic program, use UID and UIE instructions in your
	seconds		STI program file.
Immediately when an	Eject a faulty bottle from	Controller Input	Create a separate program file and specify 16 inputs of an input word in
event occurs	a bottling line	Interrupt (PII)	the I/O rack. When the event(s) occurs, the controller interrupts the main
			logic program, runs the PII to completion, then resumes the main logic
			program where it left off.
			When a block-transfer instruction to remote I/O is encountered in a PII, the
			controller resumes execution of lower priority programs (main logic
			program) until the block-transfer is completed. When this occurs and you
			want your PII to run to completion before returning to the main logic
			program, use UID and UIE instructions in your PII program file.

Priority Scheduling for Interrupts and MCPs

PLC-5 controllers prioritize when fault routines, interrupts, and main control programs are executed. This prioritization is called “scheduling.” The PLC-5 controller considers some scheduling tasks to be of greater importance than others. The scheduling priority of each task is as follows (from highest to lowest):

1.Fault Routine

2.Controller Input Interrupt (PII)

3.Selectable Timed Interrupt (STI)

4.Main Control Program (MCP)

Publication 1785-UM012D-EN-P - July 2005